Theoretical Research On Continuous Variable Entanglement In Atoms And Magnons Systems

Quantum informatics as a newly subject didn't rise in the world until 1994. Thoughthe time is short, a series of important breakthroughs have been gained. Quantuminformatics mainly includes quantum information, quantum communication, quantumoptical communication, quantum computation and so on. In recent years, a surprisingprogress on quantum information has been made at the theoretic and experimental fields,which have attracted much attention of the states of all the countries and people who workin the field of science or in the information industries. Quantum entanglement, the core ofthe quantum information theory, attracts much attention from many researchers due to thefact that it is the indispensable physical resource in quantum information processing. Now,continuous variable entanglement has attracted a lot of attention in quantum optics andquantum information due to the experimental realization of quantum informationprocessings in continuous variable regime. Therefore, the preparation of continuousvariable entanglement attracts much attention now.Entanglement propertities of several continuous variable systems are studied in thisthesis, and the main research contents are as follows:First, entanglement properties of the steady states and the transient states in acascade three-level atomic system are investigated respectively according to the two-modecontinuous variable criterion. The top and bottom levels of the system are coupled by astrong external pump field. Two different frequency fields that may be entangled can beproduced by the transition from the top to the bottom level via the intermediate level. Wefind that the density of pump field and atom-field coupling constants play the positive rolein entanglement properties, while detuning, damping constant and atom decay rates playthe negative role in entanglement properties. Furthermore, we study the relation betweenquantum entanglement and violation of Bell's inequality in another three-level cascade atomic system. We find there are some states which are entangled do not violate the Bell'sinequality.Second, the dynamics of entanglement for two-mode magnons in antiferromagnetand ferromagnet is investigated respectively according to the two-mode continuousvariable criterion. We can obtain the time-dependent operators of magnons by the timeevolution operator. The quantum fluctuation expressions of the spin components can alsobe obtained. It is shown that entanglement between the two spin-waves along the oppositedirection can be generated and occurs periodically with time in the antiferromagnet, andentanglement between the two spin-waves along the same direction which has differentenergies can also be generated and occurs periodically with time in the ferromagnet.Third, entanglement properties of two-mode squeezed coherent states andsuperposition states from several two-mode coherent states in radiation field areinvestigated according to the continuous variable entanglement criteria. It shows that thesqueeze angle and squeeze parameter play an essential role in the study of entanglementproperties for two-mode squeezed coherent states. On the other hand, for superpositionstates from several two-mode coherent states, it is found that the entanglement dependsstrongly on coherent amplitude of each mode, phase angle and relative phase angle.